A solder can be broadly defined as being a fusible metal alloy composition which is used to bond material together. Solders having distinctly different properties can be obtained by using different combinations of metals and by varying the relative proportions of each metal in the solders. The most commonly used solders are binary compositions with the most widely used binary solders being comprised of tin and lead. The relative amounts of the tin and lead in tin-lead solders can be varied over wide ranges with most of the tin-lead solders which are commonly employed containing between 40 and 60 percent by weight of tin with the remainder being lead. Tin-lead solders have wide commercial applications, with lead-rich solders being employed in the plumbing arts and tin-rich solders being extensively employed in the electronic arts, such as in the wave soldering of printed circuit boards and the like.
The binary solders comprised of tin and lead, while satisfactory for many applications, are not suitable for certain specialized applications. In these applications, it has been recognized that the addition of relatively minor amounts of one or more additional metals to the tin-lead solders can impart to the resulting solder composition the properties required for the specialized applications.
One particular well-known class of modified tin-lead solders contains as an additive metal a few percent by weight of silver. A typical solder of this type is comprised of 61.5 to 62.5 percent by weight of tin, 1.75 to 2.25 percent by weight of silver, with the remainder being lead. The silver modified tin-lead solders are employed extensively in soldering silver contacts of electrical and electronic components. The silver in the tin-lead solders prevents the silver from the silver contacts of the electrical and electronic components from being leached into the solder. In addition, the silver modified solders have other desirable properties required for certain applications, such as excellent thermal and electrical conductive properties as well as excellent resistance to thermal fatigue caused by the frequent substantial changes in temperature. The wetting out and flow properties of silver modified tin-lead solders during soldering are quite good, and the silver modified tin-lead solders are also more resistant to corrosion than the binary tin-lead solders.
The silver modified tin-lead solders have not, however, proved to be completely satisfactory. One major problem encountered with the silver modified tin-lead solder is that unequal solidification is often encountered on cooling of solder joints made with silver modified tin-lead solders. When unequal solidification is encountered, the resultant solder joints will have areas with different amounts of stress which can cause mechanical failure if not corrected. To correct the problem of unequal solidification, the soldered assemblies are subjected to a series of reheating steps at elevated temperatures. The reheating process is both costly and the repeated heating to the required elevated temperatures can damage sensitive electronic components.
The tensile strength of the joints made with silver modified tin-lead solders is a further area of concern. The tensile strength of the silver modified tin-lead solder joints as formed is typically about 7000 PSI (49.21.times.10.sup.5 kg/sq m) which is marginally for many applications, especially in spacecraft applications. A further problem with the silver modified tin-lead solders is that the silver, because of its high price per ounce, adds significantly to the material cost of the solders.
What would be highly advantageous would be a solder comprised of base metals having the same or improved desirable properties as the silver modified tin-lead solders and paticularly a base metal solder having improved solidification properties and higher tensile strength as compared to silver modified tin-lead solders.